The present invention relates to a method for the verification of the calculated radiation dose of an ion beam therapy system especially operated with heavy ions.
Ion beam therapy systems are preferably used in the treatment of tumours. An advantage of such systems is that, on irradiation of a target object (target), the major portion of the energy of the ion beam is transferred to the target, while only a small amount of energy is transferred to healthy tissue. A relatively high dose of radiation can therefore be used to treat a patient. X-rays, on the other hand, transfer their energy equally to the target and to healthy tissue, so that for health reasons, for the protection of the patient, it is not possible to use a high dose of radiation.
There is known from U.S. Pat. No. 4,870,287, for example, an ion beam therapy system in which there are generated from a proton source proton beams of which the protons can be delivered to various treatment or irradiation sites by an accelerator device. Provided at each treatment site is a rotating cradle having a patient couch so that the patient can be irradiated with the proton beam at different angles of irradiation. While the patient is spatially located in a fixed position inside the rotating cradle, the rotating cradle revolves round the body of the patient in order to focus the treatment beams at various angles of irradiation onto the target located in the isocentre of the rotating cradle. The accelerator device comprises a combination of a linear accelerator (LINAC) and a so-called synchrotron ring.
In H. F. Weehuizen et al. CLOSED LOOP CONTROL OF A CYCLOTRON BEAM FOR PROTON THERAPY, KEK Proceedings 97-17. January 1998, a method of stabilising the proton beam in proton beam therapy systems is proposed in which the treatment beam is actively so controlled that it is located on the centre line of the corresponding beam delivery system at two measurement points spaced from each other in the longitudinal direction. The first measurement point is located between a pair of deflection magnets and is formed by a multi-wire ionisation chamber. Depending on the actual value of the beam position delivered from that multi-wire ionisation chamber relative to the centre point of the beam path, a PI control is generated by further deflection magnets arranged upstream from the first-mentioned pair of deflection magnets. The second measurement point is located just upstream of the isocentre and is formed by an ionisation chamber which is divided into four quadrants. Depending on the actual position value of that ionisation chamber, again PI control signals are generated, but those control signals are intended for the first-mentioned deflection magnets. Such a control arrangement is said to render possible both angle stability in terms of the centre line of the beam delivery system and lateral position stability of the proton beam.
When, however, heavy ion irradiation is carried out, that is to say irradiation with ions that are heavier than protons, large and heavy devices are necessary, with the result that there is a tendency to avoid the use of rotating cradles and instead move the patient or the patient couch. Corresponding therapy systems are described, for example, in E. Pedroni: Beam Delivery, Proc. 1st Int. Symposium on Hadrontherapy, Como, Italy, Oct. 18-21, 1993, page 434. Such systems are accordingly eccentric systems.
Since, however, mainly isocentric systems are preferred by oncologists, a heavy ion beam therapy system was proposed in which, although rotating cradles are used at the treatment sites, the radii of the rotating cradles can be reduced by virtue of the treatment beam delivered to each rotating cradle horizontally along its axis of rotation being so guided by means of suitable magnet and optics arrangements that, for the irradiation of a target, the beam is first of all directed away from the axis of rotation and later crosses the axis of rotation again in the isocentre. There is provided for the irradiation of the target a grid scanner, which comprises vertical deflection means and horizontal deflection means, each of which deflects the treatment beams perpendicular to the beam axis, with the result that an area surrounding the target is scanned by the treatment beams. Such a system thus essentially provides beam guidance in only one plane of the rotating cradle.
The irradiation by the grid scanner is carried out with the aid of radiation dose data, which are automatically calculated by the supervisory control system of the ion beam therapy system in dependence upon the patient to be irradiated or treated.
Since a high level of operational safety and operational stability in terms of the treatment beam is always necessary in ion beam therapy systems, a monitoring device for monitoring the treatment beam delivered by the grid scanner is provided in the afore-described heavy ion beam therapy system. This monitoring device is arranged between the last deflection magnet of the above-mentioned magnet arrangement and the isocentre, and can comprise ionisation chambers for monitoring the particle flow and multi-wire chambers for monitoring the beam position and the beam width.
For safety reasons, various DIN standards have to be observed in the operation of medical electron accelerators. Those standards are concerned on the one hand with the inspection test, that is, the inspection of the readiness for operation, and on the other-hand with the consistency test, that is, examination of operational stability, of the system. For ion beam therapy systems, especially for heavy ion beam therapy systems, safety standards of that kind developed specifically for such systems are not yet known, but there is still a need, in ion beam therapy systems too, for as high as possible a level of operational safety and operational stability.
The problem underlying the present invention is therefore to propose a method for the verification of the calculated radiation dose of an ion beam therapy system in order to improve the operational safety and operational stability, especially in respect of the irradiation planning. The method shall at the same time be suitable especially for use with heavy ions.
The problem is solved in accordance with the present invention by a method having the features of claim 1. The dependent claims each define preferred and advantageous embodiments of the present invention.
According to the present invention, an ion beam therapy system is operated which comprises a grid scanner device, arranged in a beam guidance system, having vertical deflecting means and horizontal deflecting means for vertical and horizontal deflection of a treatment beam perpendicular to its beam direction so that the treatment beam is deflected by the grid scanner device to an isocentre of the irradiation site and scans a specific area surrounding the isocentre, irradiation being carried out on the basis of calculated radiation dose data. The accuracy of the calculation of the radiation dose data is verified by using a phantom, a discrepancy between the radiation dose calculated for the at least one measurement point of the phantom and the radiation dose measured for the at least one measurement point being determined and evaluated.
Different phantoms can be used for different media to be irradiated.
It is proposed, especially, that the calculated radiation dose values be checked for a plurality of measurement points in the phantom, it being concluded that the accuracy of the calculation of the radiation dose data is adequate if the average discrepancy, for all the measurement points, between the calculated and the measured radiation dose values does not exceed a predetermined first tolerance value and, for each individual measurement point, the discrepancy between the radiation dose calculated and measured for that measurement point does not exceed a predetermined second tolerance value, the first tolerance value being xc2x15% and the second tolerance value being xc2x17%.
In order to check that the geometric structures at the treatment site and planning parameters are correctly transferred from an image-forming device of the ion beam therapy system to the positioning means, a digital reconstruction, especially an X-ray reconstruction, of the phantom can be calculated, which is compared with an X-ray image taken of the phantom, in order to ascertain a possible discrepancy.
The present invention renders possible a clear improvement in the operational stability and operational safety of the ion beam therapy system and defines a checking plan having particular checking aspects that can be performed in the sense of an inspection test and/or a consistency test of the ion beam therapy system. This relates especially to the irradiation planning, in the course of which radiation dose data are calculated in the ion beam therapy system in dependence upon the patient being irradiated or treated.